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SubjectRe: [RFC PATCH V2 01/12] fs/stat: Define DAX statx attribute
On Wed, Jan 15, 2020 at 9:39 PM Darrick J. Wong <darrick.wong@oracle.com> wrote:
[..]
> > attempts to minimize software cache effects for both I/O and
> > memory mappings of this file. It requires a file system which
> > has been configured to support DAX.
> >
> > DAX generally assumes all accesses are via cpu load / store
> > instructions which can minimize overhead for small accesses, but
> > may adversely affect cpu utilization for large transfers.
> >
> > File I/O is done directly to/from user-space buffers and memory
> > mapped I/O may be performed with direct memory mappings that
> > bypass kernel page cache.
> >
> > While the DAX property tends to result in data being transferred
> > synchronously, it does not give the same guarantees of
> > synchronous I/O where data and the necessary metadata are
> > transferred together.
>
> (I'm frankly not sure that synchronous I/O actually guarantees that the
> metadata has hit stable storage...)

Oh? That text was motivated by the open(2) man page description of O_SYNC.

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